1. Field of the Invention
The present invention relates to an automatic transmission and, more particularly, to a planetary gear unit that is suitable for use as a forward-reverse switching mechanism of a continuously variable automatic transmission. More specifically, the present invention relates to the structure of a carrier of a planetary gear unit in which a hub for a frictional engagement element such as a brake or the like is integrated with the carrier.
2. Description of the Related Art
In general, a belt-type continuously variable automatic transmission includes a belt-type continuously variable speed-change unit and a forward-reverse switching mechanism. The forward-reverse switching mechanism may employ either a planetary gear unit having a single pinion carrier (hereinafter referred to as a simple planetary gear unit) or a planetary gear unit having a dual pinion carrier (hereinafter referred to as a dual planetary gear unit). The forward-reverse switching mechanism employing a simple planetary gear unit has a ring gear coupled to an input shaft, a sun gear coupled to a primary pulley, a forward clutch interposed between the ring gear and the sun gear, and a carrier selectively stopped by a reverse brake.
Heretofore, a forward-reverse switching mechanism of the above-described type has had a construction wherein which a rear lateral plate of a carrier (a carrier cover) is extended axially forward on a radially outward side of the planetary gearing, a drum-type cylindrical member is extended to surround the outer periphery of a forward clutch, and a plurality of friction plates are alternately disposed between the cylindrical member and the case so as to form a reverse brake (e.g., see Japanese Patent Laid-Open Publication No. 06-221384). It should be noted herein that the “front side” (engine side) of an automatic transmission is connected to an engine and the “rear side” of the automatic transmission is the side opposite the “front side.”
Since the forward clutch and the reverse brake axially overlap with each other, the aforementioned forward-reverse switching mechanism can be reduced in its axial dimension but is correspondingly bulky in its radial dimension. In addition, one lateral plate (a carrier cover) having the cylindrical member constituting a brake hub and the other lateral plate (a carrier body) that is annular in shape are integrated by a bridge, to form the aforementioned carrier. In mounting a pinion on the carrier, the pinion is installed between the lateral plates by passing the pinion through a space between the cylindrical member and the annular plate, a troublesome and difficult operation. Further, a space is required to allow the pinion to be passed between the cylindrical member and the annular plate, which also leads to an increase in radial dimension.
During forward movement of the vehicle, the forward clutch is engaged and the planetary gear unit rotates integrally. Therefore, the aforementioned forward-reverse switching mechanism does not require a large amount of lubricating oil. On the other hand, during a reverse movement of the vehicle, the forward clutch is released and the reverse brake is engaged so that the carrier is stopped, and rotation of the ring gear transmitted from the input shaft is transmitted to the sun gear via the pinion as decelerated and reversed rotation. Therefore, the forward-reverse switching mechanism requires a larger amount of lubricating oil during reverse movement of the vehicle than during forward movement. Because the forward-reverse switching mechanism is constructed as a cylindrical member, a lubricating oil passage cannot be formed in one lateral plate. Lubricating oil is intended to flow from a radially extending oil hole formed in the sun gear due to centrifugal force and to thereby lubricate the pinion and the like but, instead, the lubricating oil exiting the oil hole is first scattered about in the space and then supplied to a gear meshing face. Therefore, the meshing face or the like may not be supplied with a sufficient amount of lubricating oil.
Lubricating oil urged by centrifugal force is supplied to the forward clutch and the reverse brake. For this reason, a plurality of through-holes need to be formed in the clutch drum and in the aforementioned cylindrical member serving as a brake hub. This leads to an increase in manufacturing cost.